2. Field of the Invention
The invention relates to an optical fiber, and more particularly to improvements in an optical fiber which is used in an optical communication system and transmits only specified transmission modes and which is constructed of three layers consisting of a core, an intermediate layer, and a cladding layer.
2. Description of the Prior Art
Owing to the advancements of transmission theory and manufacturing techniques for optical fibers, is has become possible to obtain an optical fiber whose transmission loss is several decibels per km in the transmission of optical waves in the wavelength region of visible and near infrared wavelengths.
When such an optical-fiber is used as a transmission line, the repeater spacing can be made much longer than in a conventional coaxial system. An extension of the repeater spacing, however, brings about the problem of the transmission bandwidth of the fiber. In ordinary multi-mode fibers, having a large number of propagation modes, the transmission bandwidth is limited solely by a wave distortion based on the difference of the group velocities of the respective modes, and it becomes narrower as the transmission length becomes greater.
Accordingly, for transmitting optical signals which have a certain bandwidth, the transmission bandwidth of the optical fiber must be expanded with an increase in the transmission length.
As an effective expedient for solving such a problem, the so-called graded-core fiber has hitherto been known. That is, the refractive index in the radial direction of the fiber is decreased continuously from the center towards the outer periphery. Theoretically, the graded-core optical fiber can make the group delays of the propagation modes substantially equal. In actuality, however, the radius of the optical fiber is definite and a parabolic continuous refractive index distribution is cut at the periphery of the fiber. Therefore, higher order propagation modes close to the cut off condition become smaller in the group delay than do the other modes. This becomes a factor for bandwidth limitation. In order to obtain a wide bandwidth of the graded-core optical fiber, accordingly, the propagation modes close to the cut off condition must be removed. As an optical fiber fulfilling this requirement, there has been known on e which is composed of a core disposed at the center and having a parabolic refractive index distribution and a cladding layer disposed around the core and having a refractive index higher than the lowest refractive index of the core and lower than the highest refractive index at the center of the core, or one which an intermediate layer having a refractive index equal to the lowest refractive index is arranged between the core and the cladding layer (see U.S. Pat. No. 3,785,718 for example).
With the improved graded-core optical fiber, a sufficient loss must be bestowed on the higher order modes, and it is necessary to render negligible the influence on the transmission bandwidth by the higher order modes whose group delays are smaller than those of the other modes. Concretely, the unnecessary modes are made "leaky" by the three-layer structure as discussed previously.
The loss of the leaky modes is determined by the refractive indexes and thicknesses of the layers constituting the optical fiber. In practice, it is desirable to make the refractive index of each layer and the core radius constant and to control only the width of the intermediate layer independently of them.
In general, when the width of the intermediate layer is large, the loss of the leaky modes becomes small, and when it is small, the loss becomes large. In order to optionally set the loss of the leaky modes in this manner, a structure is required in which the width of the intermediate layer can be freely changed. The prior-art optical fiber, however, has the disadvantage that once the refractive index distribution of the core has been determined, the design of the intermediate layer lacks versatility.